Paint brush bristles made of synthetic materials have been known for many years. It has long been known that the best synthetic paint brush bristles are those which are tapered, and the Olmer U.S. Pat. No. 2,666,976 is an early patent showing the manufacture of synthetic tapered brush bristles, in this case formed from a solution of acrylonitrile polymer. This patent also mentions that it is desirable for the bristles to have internal void spaces or to be hollow. However, for a variety of reasons which will be pointed out below, it has not heretofore been possible to make satisfactory tapered hollow brush bristles from synthetic resins.
The paint brush manufacturing industry now uses a wide variety of synthetic bristles, in addition to natural bristles. These are made of various materials including, most importantly, polyester and nylon. Until recently, the synthetic filaments were of the solid (not hollow) type, and these were provided in both the level (untapered) and tapered shapes. More recently, commercial level hollow bristles have come into the marketplace, but still the filament manufacturing industry has not been able to provide the paint brush industry with the desired tapered, hollow bristles. Very recently, one manufacturer has managed to bring into the market a tapered bristle having the cross-section shown in FIG. 2 of the Champaneria U.S. Pat. No. 3,745,061, but this is still not the configuration most desired by the paint brush industry.
Tapered filaments are desired for paint brush bristles in order to give desired properties to the completed paint brush; important among these properties is the "snap-back" of the brush (i.e., this is a certain kind of stiffness in the brush which ensures that after the brush is deformed from its normal straightness, it snaps back to its usual position). The paint brush industry also demands a different type of stiffness in their brush bristles, namely a relatively high stiffness-to-weight ratio. The reason that hollow filaments are in demand is because they have a higher stiffness-to-weight ratio.
Another reason hollow filaments are desirable is that the ends of paint brush bristles are flagged (i.e., split) so that the tip of the paint brush will deposit paint more evenly and uniformly. Hollow bristles, in general, produce more uniformly flagged ends and less fraying of the flagged end portions.
Even though the paint brush industry has long demanded a hollow, tapered synthetic bristle (because it was postulated that such a bristle would maximize the desired properties), the filament manufacturing industry had been unable to provide such a structure for a number of reasons. In the first place, it has been difficult to produce level, hollow bristles, and when it is then necessary to taper the bristle, the difficulties become compounded. Early efforts have been mixed and inconsistent, suitable fibers sometimes being obtained and other times not being obtained; such inconsistency is unsuitable for commercial production.
Two very important defects have been noted in attempts to produce tapered, hollow bristles. These are the tendency of the filament to either flatten, as shown in FIG. 1A, or fail to form a circular cross-section, as shown in FIG. 1B, the resultant cross-section in the latter case being similar to the numeral 6 or its mirror image, as shown in FIG. 1B. Both of theses undesirable cross-sections have repeatedly occurred in spite of all efforts to obtain the circular cross-section desired. It must be remembered that the hollow bristles are very small and, therefore, procedures such as shown by the Sheridan U.S. Pat. No. 2,940,126, relating to the manufacture of tapered medical-surgical tubes, are not satisfactory as one cannot use Sheridan's circular die opening because of the small dimensions involved. In other words, while a mandrel may be used when making large tubes, such as shown in the Sheridan patent, one cannot use such a device in making bristles because the dimensions are so small, and because the dies involved must, for economical reasons, have up 800 or more orifices. Supporting 800 tiny mandrels within 800 circular orifices of small dimension, and maintaining the alignment, is not feasible.
It has, accordingly, been proposed in a number of patents (note, for example, van Drunen et al U.S. Pat. Nos. 3,323,168; Opfell 3,558,420; Shimoda et al 3,600,491) to provide split, or less than fully circular, orifices for the spinning of hollow filaments. Yet, it is these types or orifices which have produced inconsistent results leading to structures such as shown in FIGS. 1A and 1B, noting, for example, FIG. 8 of Shimoda et al and FIG. 6 of the Bishop et al U.S. Pat. No. 3,340,571. The FIG. 4 orifice of Shimoda consistently produces a collapsed structure (FIG. 1A). While many prior patents show idealized pictures of hollow filaments (e.g., Opfell U.S. Pat. No. 3,558,420, FIGS. 11 and 12; Tolliver, U.S. Pat. No. 3,772,137, FIGS. 2 and 4; Hodge, U.S. Pat. No. 3,924,988, FIGS. 3 and 5; Cox, U.S. Pat. No. 4,020,229), in fact it will be understood that following the directions of the prior art patents does not provide sufficient regularity and consistency, and two of the major problems which ensue are flattening of the filament, as shown in FIG. 1A, and failure to fuse which results in the numeral 6 configuration, as shown in FIG. 1B.
Two other major problems have occurred during prior attempts to taper the hollow bristles. The first of these is that stretching of a hollow bristle to provide a taper has a tendency to produce a bristle of inconsistent wall thickness. The second problem relates to the uniformity of the taper itself from bristle to bristle: tapered bristles are somewhat bottle-shaped, i.e. there is a neck-down location at some point along the length of the bristle where the slope of the taper is greater than at any other location. To produce a satisfactory paint brush with suitable snap-back and consistent properties, the location of the neck-down must be approximately the same in every bristle in the paint brush. In past attempts, it has not been possible to make a hollow, tapered brush bristle with either consistent wall thickness or consistency of neck-down location.